Introduce the base class for WordPress xProfile fields.

BP_XProfile_Field_Type_WordPress uses [12868] improvements to specify the features & visibility the wp-biography and wp-textbox fields support. It hooks to bp_xprofile_set_field_data_pre_save introduced in [12869] to handle field values persistency. It also handles fetching field values during a profile loop and caching these field values for a latter use.

Props DJPaul, Offereins, needle, netweb, vapvarun

See #7162